Pat the chicken cutlets dry and season both sides generously with Cajun seasoning. Heat olive oil and butter in a large skillet over medium-high heat. Add the chicken and cook for 4-6 minutes per side, until golden brown and cooked through. Remove from skillet and set aside.

In the same skillet, add the diced onion and bell pepper. Sauté for 3-4 minutes until softened. Add minced garlic and cook for 1 more minute until fragrant.

Pour in the chicken broth to deglaze, scraping up any browned bits. Simmer for 2 minutes to reduce. Lower the heat, then whisk in the heavy cream, followed by the grated Parmesan cheese. Stir until the sauce is smooth, then season with more Cajun seasoning to taste.

Return the chicken to the skillet. Add the optional spinach and allow it to wilt. Simmer for 2-3 minutes to let the flavors meld. Serve immediately.